1. Field of the Invention
The present invention relates to a microcomputer, and more specifically to a microcomputer internally including a controller operating in synchronism with an external synchronous signal.
2. Description of Related Art
In the prior art, a microcomputer of this type is widely incorporated for control in a product accompanied by a "on screen display" (abbreviated to "OSD" hereinafter) in a CRT display or a television receiver. A prior art microcomputer internally including an OSD controller comprises a dedicated oscillation circuit used for only the OSD controller and another dedicated oscillation circuit used for only the other circuits including a CPU, in order to elevate an OSD display quality, or alternatively an oscillation circuit provided in common to the OSD controller and the other circuits including the CPU, in order to commonly use external terminals to reduce the number of required external terminals.
Referring to FIG. 1, there is shown a block diagram of an oscillation source portion of a first example of the prior art microcomputer. The shown prior art microcomputer is generally designated with Reference Numeral 10, and includes a dedicated oscillation circuit 12 used for only an OSD controller 14 and another dedicated oscillation circuit 16 used for only the other circuits 18 including a CPU. The oscillation circuit 12 is connected through a pair of external terminals 20 and 22 to an external oscillator (not shown) such as a quartz crystal resonator, to generate a clock signal in cooperation with the external oscillator, and also connected through another external terminal 24 to receive a horizontal synchronous signal Hsync, so that the oscillation circuit 12 is controlled by the horizontal synchronous signal Hsync. On the other hand, the oscillation circuit 16 is connected through a pair of external terminals 26 and 28 to another external oscillator (not shown) to generate a clock signal in cooperation with the external oscillator
Referring to FIG. 2, there is shown a timing chart illustrating an operation of the oscillation source portion of the first example of the prior art microcomputer shown in FIG. 1. As seen from FIG. 2, when the horizontal synchronous signal Hsync becomes active, the oscillation circuit 12 for the OSD controller stops its oscillation at once, as shown by "a" in FIG. 2, and restarts the oscillation as soon as the horizontal synchronous signal Hsync is rendered inactive, so that an output of the oscillation circuit 12 is synchronized with the horizontal synchronous signal Hsync. On the other hand, the oscillation circuit 16 for the other circuits 18 including the CPU, oscillates independently of the oscillation circuit 12 for the OSD controller 14.
Referring to FIG. 3, there is shown a block diagram of an oscillation source portion of a second example of the prior art microcomputer. The shown prior art microcomputer is generally designated with Reference Numeral 30, and includes an oscillation circuit 32 provided in common to an OSD controller 34 and the other circuits 36 including a CPU. The oscillation circuit 32 is connected through a pair of external terminals 38 and 40 to for example an external oscillator (not shown) such as a quartz crystal resonator, and on the other hand, a horizontal synchronous signal Hsync is supplied through another external terminal 42 to the OSD controller 34, but is not supplied to the oscillation circuit 32.
Referring to FIG. 4, there is shown a timing chart illustrating an operation of the oscillation source portion of the first example of the prior art microcomputer shown in FIG. 1. As shown in FIG. 2, the oscillation circuit 32 operates asynchronously with the horizontal synchronous signal Hsync. Therefore, the oscillation circuit 32 does not executes the stopping and the restarting of the oscillation, differently from the first example of the prior art microcomputer shown in FIG. 1.
The above mentioned prior art microcomputers have the following problems:
A first problem is that, in order to elevate the OSD display quality, a number of external terminals are required for the microcomputer, and a noise countermeasure of a peripheral circuit becomes complicated. An increased number of peripheral circuit parts required to compensate for shortage of the external terminal number and for conducting the noise countermeasure, results in an elevated cost and in an increased mounting area. This is hindrance in miniaturization and in a cost-down of the product.
The reason for this is that: If a time difference between a transition of the horizontal synchronous signal Hsync and a transition of the clock generated in the oscillation circuit is different from one horizontal period to another as shown in FIG. 4, a deviation occurs on the OSD, with the result that the OSD display quality is deteriorated. This deviation will be called a "horizontal deviation".
In order to prevent this horizontal deviation, the oscillation circuit is stopped and then caused to restart the oscillation at each horizontal synchronous signal, as the oscillation circuit 12 in the first example of the prior art microcomputer shown in FIG. 1.
However, if a "spike noise" occurs as shown by "b" in FIG. 2 as the result of a sudden stop of the supplying of the clock, there is possibility that the other circuits including the CPU cause malfunction. Because of this, the OSD controller 14 and the other circuits 18 including the CPU are required to have the dedicated oscillation circuits 12 and 16, respectively.
A second problem is that: If external terminals are used in common in order to reduce the number of external terminals, the OSD display quality is deteriorated. Because, if the oscillation circuit is used in common to the OSD controller and the other circuits including the CPU, the output of the oscillation circuit is not synchronized with the horizontal synchronous signal, with the result that the horizontal deviation occurs in the OSD controller as mentioned hereinbefore.